Colons can be used to join two closely related ideas, each of which could be written as a separate sentence. In this case, writers use colons to signal the reader that the part following the colon explains the part preceding the colon.

I love summer rainfall: it reminds me of playing in the rain as a child.

Colons can also be used to indicate an upcoming list or to signal that more information about something which has just been mentioned is to follow.

They had three goals: finishing, going home, and relaxing.

Colons can be used to introduce quotations.

The organization posted its new motto: “Make it easy on yourself.”

Semicolons

Like colons, semicolons can be used to join closely related ideas, each of which could be written as a separate sentence. In this case, however, the second part does not explain the first part; it just restates, reinforces, or expands upon it.

Although he incorporated the new techniques, he struggled with them; throughout his career, he was torn between acceptance and self-determination.

Semicolons can be used to join independent clauses when the second clause begins with a conjunctive adverb such as therefore, however, moreover, thus, hence, etc. Note that a comma follows the conjunctive adverb in such cases.

When they returned, they found the network down; therefore, they were unable to access the information they needed.

Semicolons can also be used to separate items in a series when one or more of the items contain commas.

Peter, Paul and Mary; The Beatles; and Count Basie were all on his list of favorites.